FDA Approves First-Ever Treatment for Barth Syndrome, Marking Milestone in Ultra-Rare Disease Therapeutics

In a groundbreaking development for the pharmaceutical industry and rare disease community, the U.S. Food and Drug Administration (FDA) has granted accelerated approval to Stealth BioTherapeutics' Forzinity (elamipretide) for the treatment of Barth syndrome. This decision marks a significant milestone as Forzinity becomes the first FDA-authorized therapy for this ultra-rare, life-limiting pediatric mitochondrial disease.

Forzinity's Approval: A Long-Awaited Breakthrough

Stealth BioTherapeutics' journey to approval has been arduous, spanning nearly a decade of research and development. The FDA's decision, announced on September 19, 2025, comes after multiple setbacks, including a previous rejection in May 2025. The approval is specifically for patients weighing at least 66 pounds (30 kg), addressing a critical need in the Barth syndrome community.

Reenie McCarthy, CEO of Stealth BioTherapeutics, hailed the approval as a "pivotal victory" that "offers hope for expedited regulatory attention to other ultra-rare diseases." The decision aligns with the FDA's recent efforts to streamline approvals for therapies targeting ultra-rare conditions, as outlined in their newly unveiled Rare Disease Evidence Principles.

Clinical Efficacy and Ongoing Research

The FDA's accelerated approval was based on data from the Phase II TAZPOWER trial, which demonstrated improvements in knee extensor muscle strength—an intermediate endpoint correlated with enhanced walking ability. While the drug initially failed to meet primary endpoints related to the six-minute walk test, the FDA deemed the muscle strength improvements as reasonably likely to predict patient benefit.

As a condition of the accelerated approval, Stealth BioTherapeutics is required to conduct a post-approval randomized, double-blind, placebo-controlled trial to confirm the clinical benefits of Forzinity. The company is also working to expand access to children weighing less than 30 kg, who are often the most severely affected by Barth syndrome.

Implications for Ultra-Rare Disease Treatment

The approval of Forzinity represents a significant shift in the FDA's approach to ultra-rare disease therapies. Dr. George Tidmarsh, director of the FDA's Center for Drug Evaluation and Research, emphasized the agency's commitment to "facilitating the development of effective and safe therapies for rare diseases."

This decision comes amid increasing pressure from patient advocacy groups, medical experts, and lawmakers for expedited review of treatments for ultra-rare conditions. The Barth Syndrome Foundation played a crucial role in advocating for Forzinity's approval, with board chair Kate McCurdy stating, "We have everything at stake in this."

As the pharmaceutical industry continues to grapple with the challenges of developing treatments for ultra-rare diseases, Forzinity's approval may serve as a template for future therapies, potentially accelerating the path to market for other much-needed treatments in this space.